It is known to produce viscoelastic fluids for use in oil well drilling and servicing, especially for the so called "drill-in" fluids used in horizontal drilling within a hydrocarbon-bearing formation. See for example the following references: "Drill-In Fluids Improve high Angle Well Production," p. 5-11, Supplemental to the Petroleum Engineer International, March, 1995; and "Soluble Bridging Particle Drilling System Generates Successful Completions In Unconsolidated Sand Reservoirs," Jay Dobson and Delton Kayga, presented at the 5th International Conference On Horizontal Well Technology, Amsterdam, The Netherlands, Jul. 14-16, 1993.
Such fluids are characterized as having a rheological profile which is shear thinning, having a high viscosity at extremely low shear rates and a low viscosity at high shear rates. Thus such fluids are pseudoplastic having a high yield stress.
This type of rheology is produced by hydrating in the fluid certain water soluble polymers. These polymers are biopolymers, i.e., microbially produced polysaccharides or heteropolysaccharides, and are well known in the art.
It is well known that polysaccharides are degraded by heat as the temperature of the fluid containing them is increased. The thermal degradation of the polysaccharides decreases the viscosity of the fluid, most especially the low shear rate viscosity which provides the fluid with its desirable characteristics as many other water soluble polymers and other materials can provide the high shear rate viscosity required.
U.S. Pat. No. 5,514,644 discloses the use of magnesia and a thiosulfate salt to enhance the thermal stability of polysaccharide-containing, saturated salt brines. It is disclosed in the article "Biopolymer Solution Viscosity Stabilization-Polymer Degradation and Antioxidant Use," Scott L. Wellington, Society of Petroleum Engineers Journal, December, 1983, pp. 901-912, that a combination of a free radical transfer agent, a sacrificial easily oxidizable alcohol, a compatible oxygen scavenger, and a sufficient brine concentration were required to stabilize xanthan gum-containing fluids. U.S. Pat. No. 4,485,020 discloses the preparation of a thermostable, viscous xanthan gum solution by heating a xanthan gum solution in the presence of at least one C.sub.1 to C.sub.10 alkyl or C.sub.3 to C.sub.10 cycloaklyl substituted primary or secondary mono- or diamine having an upper limit of a total of 15 carbon atoms at a temperature in the range of about 30.degree. to about 130.degree. C. from about 5 minutes to 120 minutes. U.S. Pat. No. 4,900,457 discloses that the thermal stability of aqueous polysaccharide solutions can be significantly enhanced by incorporating certain formate salts therein.
Thus there is a need for enhancing the thermal stability of polysaccharide-containing aqueous fluids, particularly at low concentrations of soluble salts dissolved therein.